Earth & Sky

The following individual was interviewed for today’s show. Our thanks to:

Dr. Julie Etterson
Department of Biology
University of Virginia

Interview with Julie Etterson:

ES: Please tell me about yourself and your research with the partridge pea.

JE: My name is Julie Etterson, and I co-authored the paper called “Constraint to Adaptive Evolution in Response to Global Warming” with Ruth Shaw, and this is work that I did as part of my dissertation project while I was a graduate student at the University of Minnesota. The goal of the work was to explore whether or not populations of some plant species, and I chose to work on, the partridge pea, are going to be able to evolve rapidly in response to global warming. The broader context of the work was that, people in the scientific community are reaching the consensus that climate is definitely going to change, and the rate of climate change is more rapid, by about a factor of ten, than has ever been experienced, like during periods of glacial warming and cooling. But unlike in the past, populations today are highly fragmented because of human activity, and particularly agriculture in the Great Plains, has resulted in less than 10% of the land still supporting native vegetation. And these little habitat islands area widely spaced. In the past, during glacial cycles when climate changed a lot, plants species were able to gradually track their land of climate to which they were adapted to by migration, very slow migration. However, now because these habitat patches are so far apart, and they’re completely surrounded by agricultural fields, the likelihood that they’re going to disperse, and become established, and slowly move, is less likely. And the rate of warming is a lot more rapid than what have happened in the past. And so the question is then, given the scenario, some species that are weedy, and have good mechanisms of seed dispersal, might be able still to migrate. But if not, they either adapt or evolve in the spot where they are right now or they go extinct. So the body of the work is that we’re going to assess what is the potential for evolution in these populations. And so the first question that I asked was, and this goes beyond the information that was specifically in the paper: Have populations evolved to climate change in the past? The particular climate model that I used predicts that the climate in Minnesota will be similar, in terms of water availability, to the climate of Kansas in 35 years. And so the question is: Are plants in Minnesota and Kansas actually different in terms of their adaptation to the particular climate that they’re in right now? I actually sampled plants, not just in Minnesota and Kansas, but also further south, and looked in the greenhouse, whether or not there are differences in plant adaptation, and I found that there definitely are. Southern plants are drought tolerant and Northern plants aren’t, which is pretty much to be expected. And so the gradient in climate that occurs from north to south is paralleled by a gradient in drought tolerance among populations within the single species of partridge peas from north to south. So I identified some traits that are identified with drought tolerance. And what I wanted to find out was whether there’s sufficient genetic variation within these populations for natural selection to act on to create more drought tolerant populations in the North. And so I used the tools of qualitative genetics to get at that question. And this is just really a classical genetic approach, where you look for the resemblance among relatives. And so I collected a bunch of plants from the wild. I mated them, so I knew who the mom and dad was of every seed. And then [I] planted them in the north, and then progressively more southern sites, specifically in Kansas, in southern Oklahoma. And then I looked at the family level characteristics. And so a finding, that there’s significant genetic variance for important traits, is really a positive finding. Because it suggests that natural selection has genetically based differences among individuals that it can sort through, that it can select individuals that have higher performance and gradually evolve higher drought tolerance. And so I found that there was significant genetic variance in traits that are important for drought tolerance. And in another study I characterized that there is indeed natural selection occurring on these traits, and that the strength and direction of selection differs from north to south. And so for example one of the traits that I looked at was leaf thickness. And leaf thickness is of interest in the drought study because if you have a thick leaf, light can penetrate into the depth of the leaf, and photosynthesis can occur. But you have a smaller surface area, and so you have less water loss. And so, generally, thicker leaves are more advantageous in a drought prone environment. And so what I found is that selection was favoring plants with thin leaves in the north, and thick leaves in the south. And so it seemed that the two essential components for evolutionary response to create more drought tolerant populations were there. There’s genetic variation, there’s selection happening. However there’s one piece of information that I found that made the likelihood of evolutionary response less. And that was the genetic relationship between the traits that were important. And so what I found was that, the two traits that were under selection, to increase the rate of phenological development, so more rapid reproduction, and also increase leaf number, so those were the two directions that selection was acting on those two traits. The genetic correlation between those two traits was negative. So there was positive selection on the rate of phenological development, and positive selection on leaf number, But they’re negatively related. What a negative genetic correlation suggests is that if you increase one of the traits, you decrease the other one. And so this poses a constraint to evolutionary response. And so these genetic correlation’s, or relationships among traits that are antagonistic to the direction of joint selection on those pairs of traits. And when you look through the whole dataset, I found that half of the genetic correlation’s overall were of an antagonistic nature, rather than in accord with the direction of natural selection. And so what that meant was, when you estimated a multivariate evolutionary trajectory, the whole thing would bog down because selection was trying to increase one trait, but it was being dragged by positive selection on another trait it was negatively related to, if that makes sense. And so these antagonistic genetic correlation’s among the traits constrained the whole genetic architecture of the population and make the likelihood of evolutionary response much less, or at least much slower than what it would be if these traits are independent. And so than, just as a mental exercise, I compared the predicted rate of evolution for Minnesota plants, for example, to obtain the phenotypes of Kansas plants. How many generations of selection would it take for Minnesota plants to have leaves as thick as Kansas plants? And I compared that to how long the global climate models are predicting that it’ll take for the climate in Minnesota to become like Kansas. And so over all the traits, about half the time the rate of evolutionary response was predicted to be slower than the rate of climate change. And there are some reasons that make me think that these are overestimates anyway of how many generations it’ll take for selection to produce more drought adapted populations. And so the bottom line of that work was that it appears as if the rate of potential evolution in these populations is slower than the rate at which climate is going to change. And that is worrisome. And then if you take into account that there are severe reductions in seed output when populations are moved into more southern environments, the combination of the demographic problem that will be produced because of low production of progeny, and also the slow rate of evolution that’s been predicted, that suggests that this problem should be more closely researched in other species to find out how general the pattern is, to find out whether there are other species at risk as climate changes around them, if they don’t have the potential to move to the band of climate to which they’re adapted.

ES: Are there examples of species that are evolving and keeping up with climate change?

JE: Well, this is the first study of this kind, and so there really isn’t any other species to compare it with.
There are paleontological examples of the movements of plants with glacial periods, and the finding that plants that have better mechanisms of seed dispersal move faster than those that don’t. And there’s also some evidence that the migration process itself causes evolution of tree species. For example there’s a study on lodge pole pines that looks at the morphology of the pollen grains from south to north with the idea that the northern populations are ones that reoccupied that previously glaciated regions of the species range. And they found that the pollen grains have bigger wings in the north than in southern populations. And that suggests that selection was acting on pollen dispersal as these populations moved further north, with it being more advantageous for them to be able to move further.

ES: Can you tell me a little bit about the partridge pea, and why you chose it for study?

JE: Well, it’s an annual plant, and annual plants are typically associated with disturbance. And so historically you would find it in eroding areas or where there was a lot of animal activity. And now it’s often found along roadsides and in disturbed areas of native prairies. It’s not one of the most common species in the prairie, the most common being grasses. It’s a thorb, a legume, which is somewhat uncommon as a life form in the prairie. It gets its name, partridge pea, because large birds, turkeys and partridge, pheasants use the seeds as a food source.

ES:

JE: It was a combination of chance and practicality. I needed to find a species that I could find in large enough numbers for a wide geographic range. And this happened to be one that I happened to find. It was a convenient species to use because it is an annual, and so I could do a generation in a year, which was important to be able to get this study done during the course of a dissertation. It also is just a good native prairie species, and that also made it attractive. I mean the more typical life history of prairie species, in general, is perennial. And so I think it would be very valuable for this kind of work to be done on a perennial species sometime in the future.

ES: How does a partridge adapt to changing climate conditions?

JE: The specific traits that I looked at in the paper that was published in Science are the rate at which the phenology of the plant occurs, so in the north where the growing season is short, they have rapid growth and development. In the south it’s much more protracted. The leaf thickness is one of the other traits that I looked at, and just plant size in general. But in previous studies I also looked at lots of other things, including root mass, and rate of photosynthesis. There are a number of ways, both morphological and physiological, that the plant can acquire a more drought tolerant type. I think one of the most interesting ones was actually that the plant folds its leaves in response to midday water stress. And so it’s a compound leaf. It’s got lots of little leaflets lined up on a row. And when it’s stressed at midday, the leaves fold up, which prevents a lot of water from escaping from the leaf and also prevents damage due to overheating. And I also found that that trait varied from north to south.

ES: What effect would it have on the environment if the partridge pea were to someday disappear?

JE: It’s really hard to predict, but in my opinion the loss of any species which is a component of the natural environment is a shame. It’s a loss of a piece of biodiversity that characterizes the plains. And it’s kind of hard to say what kind of repercussions that would have on both wildlife and other species. It’s a legume, so it’s a nitrogen fixer, and it increases the nutrient content of the soil for other species that co-occur with it. Birds rely on seed production for food sources. The relevant experiment to know what would happen if it were removed hasn’t been done. But any loss of biodiversity in a complex ecosystem I think is important.

ES: What can humans do to prevent this loss of biodiversity?

JE: Well, one of the things that I tried was to introduce more drought tolerant southern genotypes into a Northern population and hybridize the plants. And that, at least in the short term, was not a great idea. The populations were so genetically diverged that they had reproductive problems like male sterility. They were just too different, they couldn’t interbreed. But I think that that kind of work – I also chose very extreme populations for the experiment ? more work of that kind should be done with human facilitated movement of southern plants with the band of climate to which they’ve historically occurred with, if you know what I mean. People moving the plants to keep pace with climate is not possible, and studies indicate that the plants are adapted to a particular range of climates and that they won’t persist outside of that range.

ES: Is there anything else that you’d like to add for the listeners of Earth & Sky?

JE: More work of this kind should be funded and done!